Alkali uranyl phosphate phosphors coactivated with rare earths

ABSTRACT

CONDENSED ALKALI URANYL PHOSPHATE PHOSPHORS COACTIVATED WITH RARE EARTH IONS WHEREIN THE ALKALI METALS ARE CHOSEN FROM THE GROUP LITHIUM, SODIUM, POTASSIUM, RUBID IUM AND CESIUM AND THE RARE EARTH COACTIVATORS ARE CHOSEN FROM THE GROUP EUROPIUM, SAMARIUM AND NEODYMIUM. THESE PHOSPHORS, WHEN EXCITED BY ULTRAVIOLET, ELECTRON, ION OR X-RAY RADIATION, EXHIBIT DOMINANT EMISSION COLORS WHICH MAY BE VARIED FROM GREEN TO RED THROUGH WHITE DEPENDING UPON THE COACTIVATOR SELECTD AND THE MOLAR CONCENTRATIONS. THE PHOSPHORS, MAY BE UTILIZED AS COLOR CORRECTORS FOR MERCURY LAMPS, IN CATHODE RAY TUBE SCREENS INCLUDING SCREENS FOR COLOR TELEVISION TUBES, AND IN DETECTORS FOR ION OR X-RAY BEAMS.

United States Patent Oflice Patented June 22., 1971 3,586,634 ALKALI URANYL PHOSPHATE PHOSPHORS COACTIVATED WITH RARE EARTHS Frank J. Avella, Flushing, N.Y., assignor to General Telephone & Electronics Laboratories Incorporated N Drawing. Filed Feb. 19, 1968, Ser. No. 706,662 Int. Cl. C09k N30 US. Cl. 252--301.1 10 Claims ABSTRACT OF THE DISCLOSURE Condensed alkali uranyl phosphate phosphors coactivated with rare earth ions wherein the alkali metals are chosen from the group lithium, sodium, potassium, rubidium and cesium and the rare earth coactivators are chosen from the group europium, samarium and neodymium. These phosphors, when excited by ultraviolet, electron, ion or X-ray radiation, exhibit dominant emission colors which may be varied from green to red through white depending upon the coactivator selected and the molar concentrations. The phosphors, may be utilized as color correctors for mercury lamps, in cathode ray tube screens including screens for color television tubes, and in detectors for ion or X-ray beams.

BACKGROUND OF THE INVENTION Field of the invention This invention relates to a group of fluorescent materials which emit light when exposed ot ultraviolet, electron, ion or X-ray radiation. In particular, it relates to condensed alkali uranyl phosphate phosphors coactivated with rare earth elements which exhibit dominant emission colors varying from green to red through white.

Description of the prior art Luminescent phosphate compositions containing uranyl ion as an activator have been reported in the literature. Nichols and Slattery in a paper titled Uranium As An Activator, Rev. Sci, Inst. 12, 449 (1926) report the photo-and cathodoluminescence of uranyl ion in condensed phosphate system s referred to as NaPO and KPO the former being prepared by fusing microcosmic salt (NaHNH PO -4H O). Dobrolyubskaya, as reported in Zh. Neorg. Khim. 28, 2004 (1963), investigated uranyl fluorescence in the condensed phosphates of sodium and concluded that the (NaPO 3 ring structure was suitable as a host.

SUMMARY OF THE INVENTION I have discovered that visible and infrared luminescence may be obtained under various excitation conditions from selected condensed alkali uranyl phosphate compositions coactivated with certain elements of the rare earth lanthanide series. More specifically, my invention relates to phosphors defined by the formula where M is an alkali metal selected from the group consisting of Li, Na, K, Rb and Cs; R is a rare earth metal selected from the group consisting of Eu, Sm and Nd; and a, b, c, and d are molar concentrations. As indicated by the designation U0 the uranium is added and incorporated in the hexavalent state, and the actual luminescent species in the phosphor is the uranyl ion, UO as evidenced by its characteristic green fluorescence. The phosphate matrices disclosed herein are referred to as condensed to indicate that there is a sharing of oxygen atoms between two or more (PO5 tetrahedra to form chain or cyclic structures. Thus, the alkali orthophosphates (which would be defined by the above formula if a=3, d=1 and b=c=0) are excluded. In the condensed phosphates of the present invention a/ d is less than 3, the ratio of a to d being approximately unity for the more etficient materials.

It has been found that efficient green-emitting phosphors result when M is selected from the subgroup consisting of Li, Rb, or Cs or a combination of alkali metals, b is in the approximate range 0.01 to 0.40, 0:0 and a/d=1. Further, when M=Li, Na, K, Rb or Cs, b=0.01-0.40, c has a value 0.01 to 0.09, a/d is less than 3, and R is selected from a rare earth activator in the group Eu, Sm and Nd, fluorescence results from both the U0 and the rare earth ions. The combination of Eu or Sm and UO produces several spectral bands or lines at Wavelengths in the range 470 to 640 nanometers producing a range of emission colors depending on the molar concentration of each activator. The combination of U0 as the activator and Nd as coactivator produces emission bands in the infrared spectral region between 870' and 1070 nanometers.

If the subject phosphate matrices are prepared without the uranyl ion but contain the rare ion, their photoluminescence spectra exhibit various excitation bands. The incorporation of uranyl ions introduces a broad excitation band for the rare earth emission which extends approximately 220 to 450 nanometers and peaks near 320 nanometers. This broad excitation band permits more eflicient excitation of the rare earth emissions by permitting utilization of a greater proportion of the exciting energy available from broad band emitters. This is significant for mercury lamp applications since the resulting phosphor responds in the region where mercury vapor luminesces.

The alkali uranyl rare earth phosphate phosphor may be prepared in powder form by mixing U0 and a rare earth oxide with one or more alkali phosphates and treating the mixture at elevated temperatures. Suitable alkali phos phates are the monobasic alkali orthophosphates analogues of microcosmic salt NaHNH PO.,-4H 'O, the metaphosphates (MPO pyrophosphates M P O where n is equal to or greater than unity. Best results have been obtained when a uranyl salt, such as uranyl nitrate which is thermally decomposable to U0 is used as a source of U0 It has been found that in some cases a small addition of P 0 in the form of (NHQ HPQ, improves the luminescence of the phosphor.

The mixture is thermally treated by firing in air in a suitable container at a temperature between 400 C. and the melting point of the phosphate for between /2 and 10 hours. Firing may be completed in a single step or in several steps, the material being mortared between each firing step. It has also been found possible in some cases to fuse the mixture and then anneal it at a temperature below the melting point thereof.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Example I A first sample of sodium uranyl europium phosphate was prepared by dry-blending 4.00 grams of NaH PO H2O 0.191 gram (NH 'PO 0.146 gram UO (-NO -6 H 0 and 0.153 gram Eu O and firing the blend in air in a fused silica crucible. After an initial prefiring Warm-up of about 30 minutes, the blend was fired at 525 C. for one hour. The sample was then allowed to cool to room temperature, mortared and then re-fired at 555 C. for one hour.

The resulting phosphor Na O -0.20(UO 0.03 (Eu O 1.05 (P 0 exhibited an orange emission under excitation by ultraviolet radiation, electrons, X-rays or ion bombardment. In particular, when excited with radiation from a low pressure mercury vapor lamp having its greatest energy output at a Wavelength of 253.7 nanometers, the phosphor exhibited a luminosity value 90% that of europium activated yttrium vanadate YVO :Eu, a phosphor used commercially as a color corrector for high pressure mercury lamps and as the red-emitting phosphor in color television tubes. When compared with another known phosphor, europium activated yttrium oxide Y O :Eu, the emission of the new luminophor under low pressure mercury vapor lamp excitation had a luminosity value 123% that of the Y O :Eu. In response to radiation from a medium pressure mercury lamp having its peak emission in the group of spectral lines near 365 nanometers, the sodium uranyl europium phosphate material exhibited a luminosity value 131% that of YVOpEu and many times greater than Y O :Eu. In all tests, visible radiation from the loW and high pressure mercury vapor lamp sources was removed with a Corning 7-54 filter.

The relative luminosity of the red component of the emission from the new phosphor was determined by measuring the luminescence transmitted through a filter (Corning 2-63) which passes only light of wavelengths 580 nanometers or greater. The luminosity value of the red component was found to be 71% that of YVO :Eu and 108% that of Y O :Eu when excited by radiation from a low pressure mercury vapor lamp. Under excitation by a medium pressure mercury vapor lamp the luminosity value of the red component was 108% that of YVO zEu. No meaningful comparison was possible with the Y O z Eu since its emission was insignificant under medium pressure excitation.

The above data indicate that the sodium uranyl europium phosphate is relatively efficient under excitation by both low and medium pressure mercury vapor lamps. Its broad excitation band pea-ks closer to the 365 nanometer output of the medium pressure lamp than does the YVOgEu excitation band. Thus, it is more elficient with that excitation source.

A second sample having the same composition was prepared using the same firing schedule except that the second firing was carried out for two hours rather than one hour. The resulting phosphor has characteristics similar to that of the first sample except that its fluorescence is somewhat more reddish in color.

A third sample was prepared using the same firing schedule as was employed with the first sample; however, 0.459 gram Eu O was used instead of 0.153 gram. The resulting phosphor, which has the formulation Na O 0.02(UO 0.09 (1511 1.05 (P 0 exhibits a greenish-white fluorescence when excited by the low and medium pressure mercury vapor lamp.

Fourth and fifth samples of sodium uranyl europium phosphate were prepared using the same firing schedule as for the first and third samples. The fourth sample differed from the first in that 0.306 gram Eu O and 0.382 gram (NH HPO were used in place of the quantities disclosed for preparation of the first sample. The resulting phosphor Na O-0.20(UO )-0.06(Eu O -1.10(P2O5) exhibited a bright orange luminescence. The fifth sample also emitted orange luminescence. It differed from the first sample only in that 2.92 grams UO (NO -6H O was used resulting in the formulation Na O- 4 Example II A sample of cesium uranyl europium phosphate having the formula C820 0.02(UO 0.03(Eu O 1.05 (P 0 was prepared by blending 1.00 gram CsH PO 0.0696 grams (NH4)2HPO4, gram UO2(NO3)2'6H2O and 0.0279 gram Eu O and then firing the blend at 525 C. for one hour. The blend was cooled, mortared and then refired at 525 C. for two hours. The resulting phosphor exhibited a bright orange luminescence under low and medium pressure mercury lamps.

Several samples of cesium uranyl phosphate phosphor were prepared. One such phosphor Cs O-0.400(UO r 0 was made by blending 2.00 grams CsHPO 0.139 gram (NH HPO 1.06 grams UO (NO -6H O and firing the blend at 525 C. for one hour followed by firing at 575 C. for one hour. Between firings the phosphor was cooled and mortared. Comparing the emission intensity of the material under a low pressure mercury lamp to a standard green-emitting phosphor, Zn SiO :Mn, it was found that the luminosity value was 108% that of the standard. When excited by a medium pressure lamp and using ZnSzCu as the standard (since ZnSiO :Mn is a poor emitter at 365 nanometer excitation), the luminosity value was found to be 111% that of the ZnSzCu standard.

Example III Two samples of rubidium uranyl europium phosphate phosphor were prepared by firing a blend at 525 C. for one hour and then refiring after cooling and mortaring at 575 C. for one hour. The blend for the first sample consisted of 1.00 gram RbH PO 0.0362 gram 0.0775 gram UO (NO -6H O and 0.0289 gram Eu O resulting in the formulation This material exhibited a red fluorescence.

The second sample differed from the first in that 0.0724 gram (NH HPO.; and 0.275 gram UO (NO 6H O was substituted. The resulting phosphor Rb O-0.20(UO -0.03 B11 0 1.lO(P O emitted green light when excited by the mercury vapor lamps.

A sample of rubidium uranyl phosphate Rb O-0.200 (U0 -P O was synthesized by blending 1.50 grams RbH PO 0.109 gram and gram UO2(NO3)2'6H2O and firing the blend at 525 C. for one hour. After cooling and mortaring, the blend was refired at 575 C. for one hour. Compared under excitation from a low pressure mercury lamp, the luminosity value for the phosphor was approximately at 78% that of ZnSiO4:Mn. Under medium pressure lamp excitation the luminosity was about that of ZnSzCu. A second sample of Rb203 P205 exhibited similar luminescence.

Example IV A sample of potassium uranyl eropium phosphate phosphor was prepared by dry-blending 4.00 grams KH PO 0.148 gram U0 (NO -6H O and 0.155 gram Eu O The blend was fired by drying it at C., heating slowly to 400 C., and then holding the blend at that temperature for one hour. The resulting formulation K 0 0.02(UO -0.03 (Eu O -P O exhibited orange-green luminescence.

A second sample of potassium uranyl europium phosphate was prepared by the same method except that 1.48

5 gram UO (NO -6H O was used in lieu of the 0.148 gram of the first sample. The resulting phosphor K20 (B11203) P205 also responded to excitation by the mercury vapor lamp by fluorescing in the orange and green spectral regions.

Example V Two samples of lithium uranyl europium phosphate phosphor were prepared using the firing schedule of Example IV. In the first sample, the blend consisted of 2.00 grams Li PO 4.57 grams (NH HPO 0.260 gram UO (NO -6H O and 0.274 gram Eu This phosphor, Li O-0.02(UO- -0.03Eu O -P O luminesced in the orange nad green regions of the spectrum when excited by the mercury lamps. The second sample was similar to the first except that 2.60 grams UO (NO -6H O was used in the blend for forming the composition L120 0.20(UO -0.03Eu O -P O having a green luminescence under the same excitation conditions.

Samples of lithium uranyl phosphate and lithium sodium uranyl phosphate were also prepared. The lithium uranyl phosphate phosphor Li O-0.066(UO )-P O was formulated by blending 1.50 grams Li PO 2.00 grams (NH HPO and 0.651 gram UO (N0 -6H O and then firing the blend at 400 C. for 2 hours. It was then cooled, mortared and refired at 525 C. for 2 hours. The phosphor exhibited green luminescence.

The lithium sodium uranyl phosphate phosphor was prepared by mixing a blend of 3 .80 grams NaH2PO4 H 0.0554 gram Li PO 0.223 gram (NH HPO 2.92 grams UO (NO -6H O and firing the blend for 2 hours at 500 C. and then refiring for 2 hours at 525 C. Using a Zn SiO zMn as a standard it was found that lithium sodium uranyl phosphate exhibits 110% of the luminosity thereof. When excited by a medium pressure mercury vapor lamp and using ZnS:Cu as the standard, the relative luminosity of the new composition was 98% that of the ZnS:Cu standard.

Example VI A sample of sodium uranyl samarium phosphate phosphor was prepared by blending 4.00 grams NaH PO -I-I O, 0.191 gram (NH HPO 1.45 grams UO (NO -6H O and 0.0506 gram Sm and firing the blend at 525 C. for one hour. After cooling, the blend was mortared and refired at 525 C. for one hour. The resulting phosphor Na O- 0.20(UO -0.01Sm O -1.05P O emitted orange luminescence under the mercury lamp excitation.

A sample of sodium uranyl neodymium phosphate phosphor was prepared by the same method as the samarium activated material except that the blend contained 0.0488 gram Nd instead of Sm. The resulting phosphor Na O-0.20(UO )-0.01Nd O -1.05(P O emitted in the green and infrared regions of the spectrum.

As many changes could be made in the above described compositions and processes it is intended that all matter contained therein shall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

1. A phosphor composition having the formula Where M is an alkali metal selected from the group consisting of Li, Na, K, Rb and Cs, R is a rare earth metal selected from the group consisting of En, Sm and Nd, a is equal to one, b is in the approximate range 0.01-0.04, c is in the approximate range 0.01-0.09 and the ratio a/ d is less than 3.

2. A phosphor composition as defined by claim 1 wherein M is Na, R is Eu, b is in the approximate range 0.02- 0.40, c is in the approximate range 0.03-0.09 and a/d is less than 3.

3. A phosphor composition as defined by claim 1 wherein M is Cs, R is Eu, b is approximately 0.02, c is approximately 0.03 and a/ d is less than 3.

4. A phosphor composition as defined by claim 1 wherein M is Rb, R is Eu, b is in the approximate range 0.02- 0.20, c is approximately 0.03 and a/d is less than 3.

5. A phosphor composition as defined by claim 1 wherein M is K, R is Eu, b is in the approximate range 0.02- 0.20, c is approximately 0.03 anl a/d is approximately unity.

6. A phosphor composition as defined by claim 1 wherein M is Li, R is Eu, b is in the approximate range 0.02- 0.20, c is approximately 0.03 and a/d is approximately unity.

7. A phosphor composition as defined by claim 1 wherein M is Na, R is selected from the subgroup consisting of Sm and Nd, b is approximately 0.2, c is approximately 0.01, and a/d is approximately unity.

8. A phosphor composition having the formula where M is an alkali metal selected from the group consisting of Li, Rb and Cs, a is equal to one, b is in the approximate range 0.01 to 0.40, and the ratio a/d is less than 3.

9. A phosphor composition as defined by claim 8 wherein M is Rb, b is in the approximate range 0.2-0.4 and a/ d is approximately unity.

10. A phosphor composition as defined by claim 8 wherein M is Li, b is approximately 0.066 and a/d is approximately unity.

References Cited UNITED STATES PATENTS 2,470,449 5/ 1949 Williams 252301.1 3,163,608 12/1964 Yocom 252301.1 3,394,084 7/1968 Avella 252301.1 3,408,303 10/ 1968 Borchardt 252-30l.1 3,457,179 7/1969 Natansohn 252-3011 OTHER REFERENCES Levshin et al., Rare Earth Elements, pp. 340-349, Publ. by Acad. of Sciences, Moscow 1959.

CARL D. QUARFORTH, Primary Examiner F. M. GITTES, Assistant Examiner US. Cl. X.R. 25 2-301 .4 

